Over the last 15 to 20 years or so, there has been substantial interest in automotive-type, lead-acid batteries which require, once in service, little, or more desirably, no further maintenance throughout the expected life of the battery. This type of battery is sometimes termed a "low maintenance" or "maintenance-free battery". The terminology maintenance-free battery will be used herein to include low maintenance batteries as well. This type of battery was first commercially introduced in about 1972 and is currently in Widespread use.
A considerable amount of attention over the years has been addressed to the type of alloys used for positive and negative grids in such maintenance-free batteries. When maintenance-free batteries were first commercially introduced, the conventional automotive lead-acid battery commonly used grids made from antimony-lead alloys in which the antimony content ranged from about 3-4.5% by weight of the alloy composition. Such alloys were capable of being formed at acceptable commercial rates into battery grids by the gravity casting techniques then widely used. Moreover, the batteries made using grids of those alloy compositions had desirable deep discharge cycling characteristics.
Unfortunately, such high antimony content lead-based alloys could not be used for maintenance-free batteries. Thus, the use of such high antimony alloys resulted in the batteries having undesirably high gassing and attendant water loss characteristics. In other words, batteries with grids made from such alloys accepted high end of charge current during constant voltage overcharge so that excessive gas generation occurred. Accompanying this gas generation was loss of water from the electrolyte.
The assignee of the present invention and its predecessors in interest have been in the forefront of research relating to alloys and maintenance-free batteries. Among the patents relating to this subject are the following U.S. Pat. Nos.: 4,006,035; 4,007,056; 4,166,155 and 4,456,579.
Much commercial interest has centered around the use of calcium-tin-lead alloys for use in making grids for maintenance-free batteries. The calcium content in such alloys for positive grids has varied generally from about 0.06 to about 0.1% by weight of the alloy while the tin has generally ranged from about 0.1 up to 0.8% and even more. More typically, the calcium content in such alloys when used for making maintenance-free battery grids has been at least about 0.08% by weight or more.
Other commercial interest for maintenance-free battery grids has been directed to the use of "low antimony" lead-based alloys, viz.--alloys containing antimony contents of about 1 to about 2%, more typically about 1.5% or so. Employing such low antimony alloys generally required efforts to employ other alloying ingredients since such low antimony alloys were not capable of being made into grids at acceptable commercial rates.
Other approaches for grid alloys in maintenance-free batteries have included the use of "hybrid" alloy systems. Most typically, a low antimony, lead-based alloy is used as the alloy for the positive grids while an antimony-free alloy is employed for the negative grids. Often, the alloy of choice for the negative grids has been a calcium-tin-lead alloy or a calcium-aluminum lead alloy.
It has been well recognized over the years that lead-acid batteries are perishable products. Eventually, such batteries will fail through one or more of several failure modes. Among these failure modes are failure due to positive grid corrosion and excessive water loss. The thrust of maintenance-free batteries has been to provide a battery that would forestall the failure during service for a period of time considered commensurate with the expected service life of the battery, e.g.--three to five years or so.
To achieve this objective, the positive grids initially used for maintenance-free batteries typically had thicknesses of about 60 to about 70 mils or so. The batteries were likewise configured to provide an excess of the electrolyte over that needed to provide the rated capacity of the battery. In that fashion, by filling the electrolyte to a level above that of the top of the battery plates, maintenance-free batteries contained, in effect, a reservoir of electrolyte available to replenish the water loss, during the service life of the battery. In other words, while the use of appropriate grid alloys will reduce water loss during the service life of the battery, there will always be some water loss in service. Having an excess of electrolyte will compensate for this loss.
Over the past several years, the manufacture of such automotive lead-acid batteries, typically termed SLI automotive batteries (principally used for the starting, lighting and ignition requirements of an automobile), has gotten substantially more complex. In addition to forming battery grids by gravity casting (e.g.--the hot metal is fed into what is termed a book mold and is then allowed to solidify, the book mold providing two side-by-side grids), equipment is now commercially available by which battery grids can be continuously cast on a rotary drum grid caster. Additionally, battery grids also can be continuously formed by expanded metal techniques. For example, a rolled or wrought alloy strip or a cast strip is slit and expanded using reciprocating dies or the like and then cut into the desired width and height dimensions to form the grid with a lug.
Automobile manufacturers thus have available a variety of techniques for forming battery grids. However, the effect on performance of the batteries when such techniques are used is not understood all that well. This lack of understanding is particularly evident in view of the factors complicating current SLI battery performance requirements.
One complicating factor in attempting to provide satisfactory service life is the seemingly ever-increasing power and energy requirements demanded in current SLI automotive batteries used in modern automobiles. Many factors have contributed to the need and/or desire for such higher power and energy for such batteries. One major indication of power currently in common usage is the rated number of cold cranking amps. The number of cold cranking amps is considered in the industry as some indication of the relative power of the battery to start an automobile in cold temperature conditions.
Yet another complicating factor is the "under-the-hood" space requirements. Automobile manufacturers have significantly lessened the space available for batteries in the engine compartment. Typically, this has required that battery manufacturers provide a lower profile battery, viz.--a battery having less overall height than previously required so as to meet current aerodynamic styling needs in automobiles.
These complicating factors (i.e., a need for increased power and energy with less available space for the battery) have required battery manufacturers to alter the internal configurations to provide the needed power in a lower profile battery container. These internal alterations have typically involved increasing the number of plates used in each cell by employing battery grids with reduced thickness. For example, the number of plates in a BCI Group 24 battery has increased from about 13 to about 19 or so over the last few years while the thickness of the positive grids has decreased from about 70 to 75 mils or so down to about 45 mils and even less in some cases. The reduction in the thickness of the positive grids together with an increase in the number of plates has allowed battery manufacturers to provide Group 24 batteries having rated capacities of 875 cold cranking amps or so. Battery manufacturers currently offer batteries in other BCI sizes having rated capacities up to 1000 cold cranking amps and even more.
What has occurred in recent years is also a substantial increase in the under-the-hood temperature to which the battery is exposed in automobile service. Obviously, the under-the-hood temperature is particularly high in the warmer climates. One automobile manufacturer has perceived that, in the past three years or so, the temperature to which an SLI battery is exposed in such warmer climates has risen from about 125.degree. .F to about 165.degree. F. in new automobiles.
The specific temperature increase which is involved is not particularly important. What is important is that such under-the-hood temperatures have in fact increased. The impact of this increase in the under-the-hood vehicle service temperatures on the failure modes has been to substantially increase the occurrence of premature battery failures. The incidence of premature battery failures due to excessive positive grid corrosion has been significant.
One attempt to deal with the acute problem of relatively high under-the-hood temperatures by one battery manufacturer has been to provide a battery designed for such high temperature conditions. This recently introduced battery goes back to the use of thicker positive grids (about 70 mils or more) while using a smaller number of plates (back down to about 10 per cell). In addition, the head space in each cell is filled with hollow plastic microspheres. The use of such microspheres is perhaps to serve as a vapor barrier to electrolyte for minimizing evaporative loss of water in the electrolyte or perhaps for limiting heat transfer or the like.
What has not been appreciated in the art is the effect of all of these complicating factors and increased under-the-hood temperature on the requirements for the battery grid alloy. The overall battery requirements have drastically increased the need for a positive grid alloy that will impart, in the resulting battery, enhanced resistance to positive grid corrosion.
As is apparent from the foregoing, a considerable amount of prior work in this field has been directed to calcium-tin-lead alloys for use in battery grids. For example, U.S. Pat. No. 4,125,690 to Bagshaw et al. notes that, at calcium contents below 0.075%, the material is insufficiently hard within acceptable periods of time after grid casting and that the corrosion of the alloy is greater as the tin content increases above 1%. Bagshaw et al. found that greatly improved results were obtained with alloys having a selected composition of calcium, tin and aluminum. The calcium content range disclosed for such alloys is from 0.075 to 0.13% by weight.
U.S. Pat. No. 2,860,969 to Walsh is directed to the inclusion of cerium as a grain refiner for lead-calcium, lead-tin-calcium and lead-tin-silver-calcium alloys, which alloys can also contain a small amount of aluminum.
The calcium contents disclosed range from 0.03 to 0.1% with the silver levels exemplified being from 0.1 to 0.5% by weight.
Additionally, and more recently, silver calcium-tin-lead positive grid alloys have been employed in sealed, oxygen gas recombinant lead-acid batteries. Such alloys also contain aluminum in an amount of about 0.02 to 0.03% by weight. The calcium content ranges from about 0.09 to about 0.11% by weight while the silver content ranges from about 0.016-0.02% by weight, and the tin content ranges from about 0.5-0.75% by weight.
In spite of all the considerable work directed to maintenance-free batteries over the past several years, the complicating factors and other aspects previously discussed have created a substantial need for maintenance-free batteries that can meet the power and energy demands required and yet have an adequate service life, particularly when used in warmer climates under current under-the-hood vehicle temperature conditions. The entire environment and requirements for the battery present an extremely complicated situation which is not all that well understood.
It is accordingly an object of the present invention to provide a maintenance-free, lead-acid battery capable of satisfactory service life when exposed to relatively high temperature environments.
Another, and more specific, object lies in the provision of an alloy composition useful for making positive grids for such maintenance-free batteries.
A still further object of this invention is to provide an alloy that can be made into positive grids for such maintenance-free batteries using any one of the commercially satisfactory manufacturing methods.
Yet another object provides a positive grid alloy for such maintenance-free batteries that will impart enhanced resistance to positive grid corrosion relative to batteries using positive grids made from alloys presently being used.
An additional object of the present invention is to provide an alloy for a positive grid that may be readily formed into a positive grid without undue loss of an of the alloying ingredients.
Other objects and advantages of the present invention will be apparent as the following description proceeds, taken in conjunction with the accompanying drawings.